Deltaic Coasts

Deltaic Coasts azs2
  • What is a delta?
  • How does the morphology of a delta reflect the processes acting to shape it?
  • Why are deltas at risk?

Geologic and archeological evidence clearly indicates that a significant part of the rise of modern societies and culture can be attributed to the development of the modern world's deltas, which started forming during the slowed post-ice age sea level rise. Numerous lines of evidence indicate that well-developed societies were occupying deltas in the time frame of 4,000 to 7,000 years before present because of the many natural resources that deltaic environments generally contain. Resources such as shellfish, fish, furs, and plants were necessarily taken from the wild by our ancestors, and so many ancient societies began proximal to deltaic environments.

The term delta comes from Herodotus, a Greek historian, and philosopher who recognized the similarity of the upside-down Greek letter delta (Δ) and the shape of the Nile river delta when viewed from the south toward the north.

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Oblique view from space of the Nile River, Nile River Delta, and the Sinai Peninsula. The lower body of water is the Red Sea: an open sea to the south (left side of image) that is developing upon actively rifting continental crust at this location. Coasts on either side of the Red Sea would be considered neo-trailing coasts because of the active rifting down the center of the Red Sea rift depression. At the top of the images is the Nile River delta at the mouth of the Nile river, denoted clearly by the meandering line of vegetation (green colored land) through the otherwise barren adjacent lands (brown colored land).
Credit: NASA (Public Domain)

What is a delta?

A delta is a subaerial and subaqueous volume of sediment that has accumulated at the mouth of a river as it enters into an open body of water. The largest deltas are the product of very large river systems that are transporting large quantities of sediment. The Ganges-Brahmaputra delta is one of the largest deltaic environments on the planet and carries vast amounts of sediment to the ocean. There, the sediment is reworked by strong tidal currents of the region to form inlets and sediment ridges.

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Satellite image of the South-Asia Ganges-Brahmaputra delta, consisting of Bangladesh and the state of West Bengal, India. It is one of the world's largest deltas and covers the coastal region shown in the image. Note the brown water where rivers drain out of the tectonically uplifted Himalayan mountains of this collisional zone. The lack of distinct barrier island systems, as well as the coast perpendicular alignment of channels and islands surrounded by channels, suggests that tidal energy is a dominant process driving the evolution of this system.
Credit: NASA (Public Domain)

Delta Morphologies and Driving Processes

Delta Morphologies and Driving Processes azs2

Variations in delta morphology tell us something about the processes that cause and drive the evolution of deltaic environments. Globally, it is widely accepted that there are three end-member morphologies of deltas that reflect the relative influence of wave energy and tidal energy in the receiving basin or sediment input by the source river into the receiving basin. On a ternary plot, these three end members each represent one apex of the plot, and all deltas fall somewhere on this plot. Deltas that are primarily the result of high rates of sediment input tend to be elongated because of their rapid outbuilding associated with high rates of deposition into the receiving basin. Wave-influenced deltas have smooth, often arcuate shorelines with numerous ridges that reflect the longshore transport of river-delivered sediment by the high wave energy. Tidally influenced deltas have numerous shoreline perpendicular tidal passes and tributaries, with sediment bodies aligned parallel to the direction of tidal exchange.

Refer to caption. Examples: Sediment (Mississippi delta), tidal (fly delta), Wave (Sao Francisco delta), tidal and wave (Copper delta)
Ternary plot showing the relative influence of sediment input, wave energy, and tidal energy on delta morphology and where some of the Earth’s deltas plot within this scheme.
Credit: Galloway, 1975, as cited in Delta Morphologies and Driving Process, licensed under CC BY-NC-SA 4.0
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Satellite image of the Mississippi River delta that has built into the northern Gulf of Mexico along the south-central United States. The elongated, digitate form of the modern locus of deposition is the result of high sediment supply by the Mississippi River and low wave and tidal energy in the Gulf of Mexico. Like many other deltas, the Mississippi River is currently in a state of deterioration because of relative sea level rise and reduced sediment loads resulting from dams that capture sediment farther upstream.
Credit: Visible Earth, NASA; NASA image created by Jesse Allen, using data provided by the University of Maryland’s Global Land Cover Facility. (Public Domain)
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Satellite image of the Danube River delta where the Danube River, the largest European river, empties into the Black Sea. The delta has been occupied by humans since the end of the Stone Age. Note the generally smooth coastline of the delta and the presence of ridges (lower half of delta) and barrier islands that suggest the front of the delta has been shaped by wave processes. The ridges represent alongshore accumulation by longshore transport processes.
Credit: NASA (Public Domain)
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Rotated view of the Fly River delta of Papua New Guinea in the lower right-hand corner, where the Fly River empties into its receiving basin. The delta morphology is characteristic of tidally influenced deltas. This delta is mesotidal with a tide range of approximately 3.5 m at the mouth to 5 m farther inland.
Credit: NASA (Public Domain)

Deltas in Crisis

Deltas in Crisis azs2

During the last decade, a substantial amount of concern has arisen regarding the health of the planet's major deltas. Over-exploitation of deltaic resources by humans, the introduction of pollutants, and excess nutrients to the rivers, as well as the management of river water that feeds deltas has severely damaged the sensitive environments of many deltas. Additionally, reduced sediment loads in many deltas, because of the construction of dams, coupled with global sea level rise and/or local land subsidence has resulted in widespread loss of deltaic wetlands and fronting sandy barrier shorelines. Because deltaic plains are so heavily relied upon by humans and, in some cases, are densely inhabited by humans, there are, for some deltas, widespread efforts to try to halt coastal erosion and environmental damage.

For example, the wetlands of the Mississippi River delta have undergone substantial change during the last century, with large areas of wetlands converted to open water because of relative sea level rise and erosion by storms. The rate is just staggering, with a football field of wetlands vanishing into open water every 30 minutes! The loss of wetlands across the delta is so severe that communities and infrastructure that were once separated from the open Gulf of Mexico by wetlands are now exposed to open marine water and have become more vulnerable to the damaging effects of storm surge. As a result, the state of Louisiana has developed a series of plans to build new land and infrastructure that would help reduce the net loss of land. The staggering change between 1932 and 2011 can be seen in the two satellite images below.

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Coastal Louisiana, 2011.  Based on a NASA satellite image, gray and white areas show land and blue indicates open water. New land—mainly coastal improvements such as shoreline revetments and enriched beach areas—that built up since 1932 is shown in green.
Credit: NOAA (Public Domain)
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Coastal Louisiana, 1932. The image combines the 2011 satellite image with a U.S. Geological Survey map, in which land areas that were present in 1932 are light gray. Since the 1930s, Louisiana's coast has lost 1,900 square miles of land, primarily marshes. Comparing the two maps reveals the dramatic coastal change.
Credit: NOAA (Public Domain)

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